This invention relates to a multifunction image-forming system that applies thermal energy imagewise to a donor sheet having a thin film, that allows the imagewise thin film to be transferred from the donor sheet onto an image-receiving material by peel/transfer so as to form an area-modulated image and/or a line image such as characters, and that enables various operations such as color image formation, particularly the preparation of direct digital color proofs (DDCP) in the printing area, black-and-white image formation, platemaking, filter production and printed wiring board (PWB) fabrication. The invention also relates to an image forming system that has the multifunction image forming capability combined with a color management system for input/output (I/O) device profile matching so that an image can be formed selectively by either laser head recording in a heating mode or by thermal recording with a thermal head in accordance with the required image quality.
A color image read from originals such as photographs and pictures or color image data supplied from a given image input device is subjected to desired processing to produce a CRT display, which is then delivered as a hard copy from an image recording apparatus or processed via a plate to produce a print. Color management or image forming systems that have these capabilities are extensively used today.
To make a print, color separation films for Y, M, C and K are first prepared on the basis of color image data as supplied from an image input device; then, a plate such as a PS (presensitized) plate is exposed through those color separation films, developed and set up on a press which is operated to produce prints. Obviously, this procedure involves a number of steps. In addition, the color of the final print depends on printing conditions including the types of the paper, ink, water and printing press used, the number of screen lines used to form halftone images, and the shape of dots in halftone image.
While plates are exposed through a set of color separation films that are produced from color originals with the aid of lith films, it is common practice today to produce a color proof from the color separation films and check for any errors that may have occurred in the color separation step or to check for the need to perform color correction. To insure approximation of what can be expected on press, it is generally held preferable to prepare the color proof by forming an image on the same paper as is used to make the final print and by using pigments as colorants. Other requirements are high resolution that enables faithful reproduction of halftone image, as well as high process stability. A dry proof production method which does not use liquid developers is a technique that is particularly needed these days. Additionally, with the recent expansion of electronic systems into the pre-press area, there is a growing demand for materials and systems that are capable of direct color proofing in response to digital signals.
With a view to producing color proofs by the dry method, there has been proposed a heat transfer recording process, in which a thermal head furnished with a number of heat generating elements is pressed against an image-receiving sheet via an ink sheet and the heat generating elements are selectively activated to generate heat in response to an image signal; the ink transfer layer in the ink sheet is discretely softened to melt or vaporized to sublime so that the ink is transferred in dots onto the image-receiving sheet, whereby the image is recorded. An apparatus, commonly referred to as a thermal printer, for forming an image by the heat transfer recording process has also been proposed.
The selective heating of the ink sheet in response to an image signal in the heat transfer recording process may be effected by laser-emitted light of high-density energy rather than by the heat-generating elements in the thermal head and this method has been proposed as "laser recording" in a heating mode. Thermal recording by means of a thermal head is limited in resolution but has the advantage of permitting high-speed image recording in a simple manner; on the other hand, laser recording in a heating mode is not as fast in recording a single full frame, however, it enables image recording at high resolution.
In heat transfer recording by either a thermal head or a laser operating in a heating mode, color images can be produced by performing multiple transfer recording on an image-receiving sheet from an ink sheet or ink sheets with three colors, yellow (Y), magenta (M) and cyan (C), or four colors, Y, M, C and K (black), to produce one print. If necessary, the time of electric current application to the heat generating elements in the thermal head or to the laser, hence, the time of heat generation by the thermal head or the time of light emission from the laser, may be controlled so as to modulate the amount or area of the ink that is to be softened to melt or vaporized to sublime, whereby either the ink density or the area of halftone (dots) is modulated to produce density gradations in individual pixels.
As described above, complicated process steps and conditions are involved in the printing area and, given image data that have been subjected to desired image processing in accordance with image reading conditions (i.e., entered image conditions), individual image input devices such as image reading apparatus have unavoidable differences in characteristics or capabilities, and so do image output devices such as image recording apparatus and printers. Under the circumstances, it is desired to develop a system which, either prior to or without color proofing, produces a CRT or otherwise display of color image data that have been subjected to desired image processing so that the image quality of the finally obtainable print can be verified on CRT with high precision.
Color management systems adapted for this purpose are described in U.S. Pat. No. 4,500,919 and International Patent Publication WO 92/17982; according to the disclosed systems, many combinations of colorants in various quantities are printed and, on the basis of their colorimetric measurements, a color reproduction signal is corrected to attain a match between the color image on print and the displayed color image. Also disclosed in those patents are methods in which, following the colorimetric registration, a white point in the color image on print is brought into registry with a displayed white point, thereby assuring enhanced color matching.
Conventionally used ink sheets are either softenable to melt or vaporizable to sublime. A meltable ink sheet comprises a base carrying a transfer layer (image-forming layer) that has a colorant incorporated in a binder selected from among waxes and other low-melting point substances that are solid at ordinary temperatures and which will soften or melt upon heating. A sublimable ink sheet is different from the meltable ink sheet in that it uses a thermally sublimable substance as the binder. Hence, the transfer layer in the ink sheet, whether it is meltable or sublimable, is usually thicker than 3 .mu.m, sometimes as thick as 20-40 .mu.m, requiring considerable energy to have the binder sublime or melt with heat; further, there is a limit to the maximum image resolution that can be achieved and the edges of halftone dots may become blurred or jagged to reduce the stability of small dots.
The use of a sublimable ink sheet presents an additional problem. Since ink density, or the amount of ink sublimation for transfer, is dependent on thermal energy, it is not easy to achieve precise control over the intensity of thermal energy to be applied to the ink layer and instability will accompany density modulation. When a meltable ink sheet is used, the great film thickness of the sheet causes instability due to thermal diffusion.
Thus, with the conventional thick-film ink sheets, the effective thermal energy concentration that can be produced by laser light is limited even if the beam spot size is reduced and image resolution cannot be increased beyond a certain level. As a result, it has been impossible to switch one to another between thermal recording with a thermal head and laser head recording in a heating mode depending upon the required image quality.
If colorants are the sole factor in matching, correct color reproduction is possible with the above-described conventional color management systems. However, when producing prints, output conditions such as the type of the support for prints, the number of colors to be employed, the amount of K (black) and the number of screen lines must also be set and, what is more, the press conditions (e.g. the order of printing, printing pressure, the amount of colorant and printing speed) must be set appropriately. In fact, it has been difficult for the prior art technologies to provide good performance with high precision under versatile printing conditions.